One of the characteristics expected for an aqueous ink for a liquid jetting device is an appropriate wetting ability. The appropriate wetting ability increases the penetration speed of the ink into a recording medium, which can improve abrasion resistance or problems such as bleeding.
It is common to add a surfactant to an ink for improving the wetting ability of the ink. Especially, because a surfactant having a fluoroalkyl group has a function of significantly reducing a surface tension of a fluid to which the surfactant is added, and thus this surfactant is widely used for an aqueous ink for a liquid jetting device. However, high surface activeness easily causes formations of micelles between the surfactants. As a result, the ink containing such surfactant has a demerit such that air bubbles are easily generated (i.e. formed).
The generation of air bubbles causes various troubles in a recording system. For example, in case of a printer equipped with a movable printing head, an ink stored in a subtank is foamed by vibration caused by a printing operation, which causes troubles such as ink leakage due to errors in detection of the ink. In addition, bubbles remain on an absorption cap as a result of an ink absorbing operation for maintenance, which may cause troubles such that meniscus is destroyed the next time a maintenance operation is performed.
Patent Literature 1 discloses that a silicone defoaming agent is added to an ink containing a fluorosurfactant, as the ink containing the fluorosurfactant tends to generate air bubbles (PLT1). Here, the defoaming agent enters and is scattered in lamella layers of bubbles so that the surfactant is replaced with the defoaming agent. As a result, the bubbles disappear. In this defoaming system, the defoaming agent needs to be incompatible to the ink system. As the defoaming agent used in an aqueous system, hydrophobic silica or polyurea is generally used. However, if the defoaming agent is not dissolved, and is present as particles in the system due to its hydrophobicity, clogging of a filter may occur. For this reason, it is desirable that such defoaming agent be not added. Moreover, the addition of the silicone defoaming agent does not provide a sufficient defoaming effect.
Patent Literature 2 discloses an ink containing N-octyl-2-pyrrolidone together with a pigment and a water-soluble resin. However, surface activeness of N-octyl-2-pyrrolidone is not sufficient, and thus the resulting ink has undesirable penetration ability to a recording medium. Specifically, the ink has deteriorated fixing ability just after printing, undesirable coloring performance, and low image quality.